1. Field of the Invention
The present invention relates to a driving method for a liquid crystal display device with a liquid crystal layer having a memory mode of operation and a driving apparatus.
2. Discussion of Background
At present, TN, STN, TFT liquid crystal display elements are widely used. In these liquid crystal display elements, a display is effected by conducting usually a predetermined driving. On the other hand, a cholesteric or a chiral nematic liquid crystal having a memory mode of operation (hereinbelow, referred to as CL-LC) is noted, and liquid crystal display devices provided with such liquid crystal (hereinbelow, referred to as CL-LCD) are studied for practical use.
CL-LC held between a pair of parallel substrates has a “twist structure” wherein the direction of the liquid crystal is twisted at a constant period. There is an alignment that the center axis of twist (hereinbelow, referred to as a helical axis) is in a perpendicular direction in average to the substrates.
There is a complete planar state (hereinbelow, referred to as a PP state) wherein each helical axis of a plurality of liquid crystal domains is substantially completely perpendicular to the substrate surfaces and an incomplete planar state (hereinbelow, referred to as PL state) wherein the direction in average of each helical axis of a plurality of liquid crystal domains is substantially perpendicular to the substrate surfaces. Then, an incident circularly polarized light, which corresponds to the direction of twist of the liquid crystal layer, is selectively reflected. The wavelength λ of the selectively reflected light is substantially equal to the product of an average refractive index nAVG of a liquid crystal composition and a pitch p of the liquid crystal composition (λ=nAVG·p).
The pitch p is determined according to p=1/(c·HTP) where c is the amount of an optically active substance such as a chiral agent or the like and HTP (Helical Twisting Power) is a constant of the optically active substance. Accordingly, the selective reflection wavelength can be adjusted by the type and amount of the optically active substance. By determining the pitch so that the selective reflection wavelength of CL-LC is outside a visible region, a display becomes transparent in visual observation at the time of selective reflection and provides an operational mode of transmission and scatter.
A PP state produces a large regular reflection for incident light and extremely high reflection characteristics at a specified viewing angle. On the other hand, a PL state produces a relatively small regular reflection and high reflection characteristics at a relatively wide viewing angle.
Further, CL-LC can exhibit a focalconic state (hereinbelow, referred to as a FC state) wherein helical axes of liquid crystal domains are directed in a random direction or are aligned in a non-perpendicular direction to the substrate surfaces. Generally, the liquid crystal layer in a FC state shows a weak scattering state as a whole. There is no reflection of light having a specified wavelength as at the time of selective reflection. Further, the FC state, the PL state and the PP state exist stably even when an electric field is not applied.
FIG. 18(a) is a diagram showing a PL state and FIG. 18(b) is a diagram showing a FC state, which indicate aligning states of liquid crystal domains in a shape of gourd-shaped drum. The selective reflection wavelength in a PP state is generally given by λ=nAVG·p. The selective reflection wavelength in a PL state tends to shift to a short wavelength side in comparison with a case of the PP state because there is a distribution in the direction of the helical axis.
By providing an absorbing layer at a rear surface side in the FC state shown in FIG. 18(b), a display in a color of the absorbing layer can be obtained. Accordingly, a display having a memory mode of operation can be realized by utilizing two states: the PL state as a clear state and the FC state as a dark state (when the absorbing layer is black).
The basic construction of CL-LCD is disclosed in George H. Heilmeier, Joel E. Goldmacher et al, Appln. Phys. Lett., 13 (1968), 132 and U.S. Pat. No. 3,936,815. Further, U.S. Pat. No. 4,097,127 discloses that an intermediate state wherein a PL state and a FC state are mixed, exists stably, which can be utilized for a display.
Next, a driving method for CL-LCD will be described. In U.S. Pat. No. 3,936,815, a PL state is changed to a FC state or a FC state is changed to a PL state respectively depending on magnitudes of the amplitude of a driving voltage. In the latter case, the change is caused via a homeotropic state (hereinbelow, referred to as a HO state) in which the liquid crystal molecules are directed substantially in parallel to a voltage application direction, and accordingly, the highest voltage is required.
In CL-LC, an effective value of the waveforms of a series of applied voltages does not directly determine the state of the liquid crystal after the removal of the voltages, but the display after the removal of the voltages relies on an application time and an amplitude of a voltage pulse applied just before.
Next, a description will be given as to a matrix display in CL-LCD. It is assumed a voltage to change the liquid crystal into a FC state is VF, a lower limit voltage to change it into a PL state is Vp and an upper limit voltage which does not cause a change of the display state even by applying a voltage is VS.
In conducting a a-line-at-a-time driving, a voltage pulse having a voltage amplitude of Vr is applied to a row electrode, and in synchronism with this, a voltage pulse having a voltage amplitude of VC is applied to a column electrode. A selection pulse is applied once to each row electrode to complete a display sequence.
In the display sequence, when an ON display is selected, a voltage amplitude of (Vr+VC) is applied once to a display pixel, and in a non-selection period in the ON display, a voltage VC is applied to the display pixel. Further, when an OFF-display is selected, a voltage amplitude of (Vr−VC) is once applied to a display pixel, and in a non-selection period in an OFF-display, a voltage VC is applied to the display pixel. When a PL state is selected in an ON time, and a FC state is selected in an OFF time, the conditions of the respective voltages are as follows.Vr+VC>VP,Vr−VC=VF 
Further, it is necessary that VC<VS, so that the written state does not change. Thus, a matrix display can be effected by controlling the applied voltages as described above.
In CL-LCD, even when the number of scanning electrodes is increased, the quality of the display when display data are written is not deteriorated. Further, a driving voltage does not increase even when the number of electrodes is increased. However, the quality of the display when writing image data is poor as the number of scanning electrodes increases. Namely, when writing a state of display, selection pulses are applied to each scanning electrode in a predetermined application time. In this case, if the number of scanning electrodes is increased, a state that the scanning lines flow on the display surface is observed. Accordingly, it is necessary to shorten an application time of selection pulses depending on an increase in the number of scanning electrodes to shorten the display sequence.
When the application time of selection pulses is shortened, preferred display characteristics can be maintained by adjusting the amplitude of applied voltages in writing to change the state from an OFF-display (FC state) to an ON display (PL state). On the other hand, there is a problem in writing to change the state from an ON display (PL state) to an OFF-display (FC state). In this case, a slightly scattering state is not sometimes sufficiently obtained in the FC state, and the alignment of the liquid crystal which shows selective reflection may partly remain. Then, the written OFF-display (FC state) does not show a sufficient darkness, when a black absorbing layer is provided at a rear surface side of CL-LCD as described above.
Namely, the contrast ratio of a display is reduced. Further, a difference of light and dark occurs in a region where the previous display was in an ON display (PL state), and thereafter, an OFF-display (FC state) was written, and also in a region where the previous display was in an OFF-display, and then, an OFF-display was written plural times in series, and therefore, an uneven display is produced.
The above-mentioned problem results from an application time of a selective pulse. When the application time is shortened, it is impossible to change the state into a slightly scattering FC state in a complete sense by writing an OFF-display one time. Further, the problem is also because the optical characteristics of the written OFF-display, namely, a degree of slight scattering in an FC state or a degree of remaining liquid crystal alignment exhibiting selective reflection, change relying on the previous state.
As a result, a previously written image is often observed as a residual image. Accordingly, it is difficult to shorten the application time of the selection pulse, i.e., to increase the number of scanning electrodes while an excellent display quality is maintained.
As described above, in CL-LCD, problems occur when the volume of the display is large by increasing the number of scanning electrodes, the contrast ratio is decreased or an uneven display is produced.
In other words, it is necessary to extend a writing time to maintain the display quality when a highly precise display is to be provided. However, when the writing time is extended, scanning lines appear to flow on the display surface. Further, the following driving method is known other than the driving method as in U.S. Pat. No. 3,936,815.
SID92, Digest, P. 759-761 (1992) discloses that a pulse-like voltage is applied to CL-LC to reset the state of liquid crystal alignment before the application of voltage into PL state or a FC state. Such a driving waveform is shown in FIG. 6.
Further, U.S. Pat. No. 5,933,203 discloses a technique of applying a voltage pulse having a larger amplitude to present a HO state, and then, successively applying a voltage pulse having a smaller amplitude.
The patent publication document of EP0957394A1 discloses a resetting method for CL-LCD. After the application of a voltage pulse to render the liquid crystal to be a HO state, a voltage pulse is applied to change the liquid crystal layer into a PL state, and thereafter, a voltage pulse is further applied to change it into a FC state. In this case, a resetting time is long as a whole because there is a phase change from the HO state to the PL state which is low in changing speed. Further, flickering is produced during the resetting time because all of the pixels are once turned to a reflective display state in the PL state.
In CL-LC after erasing the previous display, either a PL state indicating a selective reflection or a FC state indicating no reflection may be chosen when the display is rewritten. However, since a HO state at an erasing time does not show reflection, a FC state which also does not show reflection after erasing can provide a natural impression, in particular in a case of using a negative display which makes the background non-reflective.
Further, “a residual image” is one problem caused by shortening the application time of a selection pulse. Such phenomenon occurs because the optical characteristics of a written OFF-state remain. Namely, the liquid crystal alignment in a FC state is influenced by the liquid crystal alignment before the phase change, and the liquid crystal alignment during selective reflection slightly remains.
Thus, in the conventional technique, it is very difficult to obtain a FC state providing the lowest reflectance when an absorbing layer is formed on a rear surface, without the remaining of selective reflection, by applying a short voltage pulse once.
Accordingly, the present invention provides a driving method capable of resetting a display in a shorter time. Namely, it is an object of the present invention to provide a driving method and a driving apparatus which suppresses the occurrence of a residual image even during high-speed writing, prevents the reduction of the contrast ratio of a display, and can provide a highly precise display of high quality.